The collection of electronic documents with intent to organize, categorize, and process for a better understanding to their content also has an extension which allows for the specification of portions of, or complete, documents to be redacted. There are many forms of text redaction, however the most commonly accepted is the placing of black boxes over the redacted text regions; additionally, if the document type contains embedded raster image data, and some or all of that image data has been specified for redaction, it is important that this receives the same appearance of redaction covering.
Prior to the handling of this process in the electronic realm, the physical world version of this would feature a person using a black magic marker, or pasting black paper, over the redacted regions of a reproduced (by photocopy, or similar technology) version of the document, and then submitting that altered document to another reproduction (by similar technology means).
Note that in the electronic realm, simply deleting text isn't good enough as it a) changes spacing and appearance and even page breaks, which can be considered as legally unacceptable modification of the evidence, b) it becomes difficult to discern where text has been removed, which makes it difficult to accurately review and record in a redaction log.
Nor is simply replacing text with “X”'s or some other character generally good enough for the same reasons, as no matter what character is selected, there is at least some chance of a natural occurrence of a large number of them appearing contiguously.